1. Field of the Invention
This invention relates to an additional information superimposition device for superimposing additional information such as anti-duplication control information on a video signal which is suitably used for a video device, for example, a recording/playback device of video signals such as a VTR.
2. Description of Related Art
VTR (Video Tape recording devices) has been popularized in daily life, and many kinds of software which can be played back on a VTR are supplied abundantly. Digital VTR or DVD (Digital Video Discs) playback devices have been available now, and provide images and sound of exceptionally high quality.
On the other hand, there is, however, a problem that this great abundance of software can be copied without restriction, and heretofore video signals on which various kinds of additional information for preventing duplication have been used for recording and transmission.
For example, a pseudo sync signal of extraordinarily high level is inserted in a vertical blanking period as a sync signal for AGC (Auto Gain Control) of a monitor receiver, or information for preventing duplication is superimposed in a specified horizontal interval of a vertical blanking period for involving analog video signal.
In the case of a digitized video signal, an anti-duplication signal or an anti-duplication control signal comprising, for example, a duplication generation control code, is added as digital data to the video signal and recorded on the recording medium, so as to prevent or control duplication of the image directly.
It is desirable for the secret additional information such as the above-mentioned information for preventing duplication that the additional information is difficult to be removed from a video signal. However, from the above-mentioned video signal on which additional information is superimposed as information on a vertical blanking period, the additional information is removed very easily by exchanging the vertical blanking period.
Additional information added to a digital video signal is played back as a separate signal from the digital video signal, the anti-duplication control signal namely the additional information is removed and not included in the analog video signal which was D/A converted and guided to an analog output terminal for monitoring the played back video signal. Therefore, even though a digital apparatus is used, in the case of analog connection in which an analog VTR is connected to an analog output terminal, the video signal is duplicated undesirably.
To prevent such undesirable duplication, direct superimposition addition of an anti-duplication control signal is considered to be a method. However, it is generally difficult that an anti-duplication control signal is added without deterioration of a D/A converted video signal and extracted in a recording device for duplication prevention control.
As an additional information superimposition method effective for both analog and digital systems which solves the above-mentioned problems without deterioration of a played back picture, the inventors of this invention previously proposed a method that additional information such as an anti-duplication control signal was spectrally spread and the additional information namely the spectrally spread anti-duplication control signal was superimposed on a video signal (refer to Japanese Patent Application No. Hei-7-339959 and Japanese Patent Application No. Hei-8-322329).
According to this method, a PN (Pseudorandom Noise) sequence code (referred to hereinafter as PN code) used as a spread code is generated with a sufficiently short period and spectrally spread by multiplying it by the anti-duplication control signal. In this way, a narrow-bandwidth high-level anti-duplication control signal is converted to a wide-band low-level signal which does not affect the video signal. This spectrally spread anti-duplication control signal is then superimposed on the analog video signal or digital video signal to be outputted as a playback output and the analog video signal or digital video signal to be recorded. The video signal to be recorded on a recording medium may be either an analog signal or digital signal.
In this method, the additional information such as an anti-duplication control signal superimposed on a video signal as a spectrally spread wide-band low-level signal, it is therefore difficult to remove the additional information such as the superimposed anti-duplication control signal.
However, it is possible to detect and use the superimposed and spectrally spread additional information by performing inverse spectral spread. The additional information such as the anti-duplication control signal is supplied consistently to the recording device side together with the video signal, and the recording device side detects the additional information and performs duplication control consistently corresponding to the detected additional information.
In some cases, additional information is additionally superimposed and added to the video signal on which the above-mentioned spectrally spread additional information has been already superimposed.
For example, it is considered that information for searching a duplication route such as a VTR device number is superimposed on an analog video signal or digital video signal as the above-mentioned spectrally spread additional information, in some cases the same additional information is already superimposed on the input video signal when the additional information is superimposed in a playback device or recording device, in such a case the spectrally spread additional information is superimposed again additionally regardless of the already superimposed additional information.
In the case that a digital video signal or analog video signal is transmitted through a transmission path having a low-pass filter characteristic, in order to compensate deterioration of the additional information due to the low-pass filter of the transmission path, in some cases the same additional information is superimposed again additionally on the way of transmission.
However, such additional superimposition of the spectrally spread additional information can cause a bright-and-dark stripe pattern on a played back picture, which is visually perceptible, because the level of the superimposed additional information becomes higher, though which level is originally sufficiently low so that the played picture is not deteriorated. Even though the additionally superimposed spectrally spread signal is not the quite same as the already superimposed spectrally spread additional information, the played back picture can be affected because the level of the same spread signal portion becomes higher.
Such perceptible additional information loses its secrecy, and the loss of secrecy is disadvantageous if the additional information is secret information such as the above-mentioned anti-duplication control signal or information for searching a duplication route.
It is the object of the present invention to provide an information addition device for superimposing and adding spectrally spread additional information on a video signal which is capable of preventing deterioration of a picture due to additional superimposition of additional information and preventing loss of secrecy of the additional information as described herein above.